The evaluation of the results of ritonavir therapy in HIV-1 infected infants and children. The incidence and prevalence of HIV-1 infection in the worldwide population is increasing. The major mode of infection is the passage of HIV-1 from mother to child. The majority of the 1,000 infants born a year with HIV will develop symptoms within the first two years of life. It is critical to evaluate novel antiviral therapies due to the fact that most of the therapies available have a limited time span because of toxicities or viral resistance. The study is being done to see how safe, how well tolerated, and what the blood levels of ritonavir are in young infants/children when it is given alone or when it's given with two other drugs, zidovudine and lamivodine. The information will help decide the proper dosage. There will be two groups of children enrolled in the study. The children in the first group will receive a lower dose of ritonavir for a few days and then the other two medications will be added. The second group will receive a higher dose of ritonavir along with the other two from the start. It is thought that the high speed at which the HIV grows causes the disease in the body to get worse. The combination of the three drugs is thought to slow the growth rate, thus not allowing the disease to worsen. The study will last for approximately two years and will involve 18-48 children, one month to two years old, and infected with HIV-1. At the onset of the study, the children will be given examinations and blood tests, urine tests and a medical history will be taken. Medication will be given and several blood tests will be done that day. Blood will be drawn on several other days that first week. Blood drawings and physical exams will take place at specific intervals for 17 months, then every eight weeks for six months. Some of the blood and/or tissue obtained as part of the study will be stored for future study.